One early technique for executing multiple operating systems on mainframes was known as a hypervisor. Hypervisors permitted multiple departmental computers to be consolidated into a single larger computer and to run multiple operating systems simultaneously through virtualization of the state of the supervisor (the term used at the time for an operating system kernel). Each of the multiple operating systems ran on a virtual machine (VM) and even if one operating system crashed, the others would continue working.
Today's high-end servers often support virtualization and some form of hypervisor technology but usually at server-class prices (up to multiple millions of dollars).
Microprocessor based platforms have supported some types of virtual machine environments in the past, but usually for supporting applications of a different operating system rather than supporting execution of multiple operating systems simultaneously. The full virtualization of personal computer through hypervisors has had significant costs in complexity and runtime performance
Alternative approaches require modification of a guest operating system to make system calls (sometimes called hypercalls, diagnose code, or paravirtualization) to a hypervisor rather than executing machine I/O instructions. Such paravirtualization calls are then simulated by the hypervisor.
Some microprocessor vendors have also introduced hardware virtualization support. For example, Vanderpool Technology (VT) developed at Intel Corporation of Santa Clara, Calif., provides architectural and instruction level support for some virtualization assistance which are otherwise inefficient or require modification of guest operating systems. In addition, some microprocessors today provide support for multithreading, simultaneous multithreading and/or multiple processor cores, which theoretically could provide for higher performance execution of multiple operating systems. To date, the advantages of these parallel hardware and virtualization features have not been fully utilized.